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Ages and Metallicities of Extragalactic Globular Clusters from Spectral and Photometric Fits of Stellar Population Synthesis Models
Spectra of galaxies contain an enormous amount of information about therelative mixture of ages and metallicities of constituent stars. Wepresent a comprehensive study designed to extract the maximuminformation from spectra of data quality typical in large galaxysurveys. These techniques are not intended for detailed stellarpopulation studies that use high-quality spectra. We test techniques ona sample of globular clusters, which should consist of single stellarpopulations and provide good test cases, using the Bruzual-Charlothigh-resolution stellar population synthesis models to simultaneouslyestimate the ages and metallicities of 101 globular clusters in M31 andthe Magellanic Clouds. The clusters cover a wide range of ages andmetallicities, 4 Myr
| Near-infrared color evolution of LMC clusters
We present here the digital aperture photometry for 28 LMC clusterswhose ages are between 5 Myr and 12 Gyr. This photometry is based on ourimaging observations in JHK and contains integrated magnitudes andcolors as a function of aperture radius. In contrast to optical colors,our near-infrared colors do not show any strong dependence on clusterages.Tables 2 and 3 and Fig. 2 are only available in electronic form athttp://www.edpsciences.org
| A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. II. The Large Magellanic Cloud
A survey of extended objects in the Large Magellanic Cloud was carriedout on the ESO/SERC R and J Sky Survey Atlases, checking entries inprevious catalogs and searching for new objects. The census provided6659 objects including star clusters, emission-free associations, andobjects related to emission nebulae. Each of these classes containsthree subclasses with intermediate properties, which are used to infertotal populations. The survey includes cross identifications amongcatalogs, and we present 3246 new objects. We provide accuratepositions, classification, and homogeneous measurements of sizes andposition angles, as well as information on cluster pairs andhierarchical relation for superimposed objects. This unification andenlargement of catalogs is important for future searches of fainter andsmaller new objects. We discuss the angular and size distributions ofthe objects of the different classes. The angular distributions show twooff-centered systems with different inclinations, suggesting that theLMC disk is warped. The present catalog together with its previouscounterpart for the SMC and the inter-Cloud region provide a totalpopulation of 7847 extended objects in the Magellanic System. Theangular distribution of the ensemble reveals important clues on theinteraction between the LMC and SMC.
| Ultraviolet ages of young clusters in the Magellanic Clouds.
Following a previous investigation on the integrated UV colours ofstellar clusters (Barbero et al. 1990), we study the calibration of theultraviolet colour index C(15-31) in terms of cluster age, usingobservations by the International Ultraviolet Explorer of 29 young andpopulous clusters of the Large Magellanic Cloud (LMC), and of the SmallMagellanic Cloud (SMC). The study is limited to the range of ages5x10^6^ to 8x10^8^yr, which is free from contamination by HorizontalBranch stars. It is shown that in this range of ages the theoreticalsequence C(15-31) vs. age agrees well with the one derived by combiningthe observed colour index C(15-31) with the ages determined viaisochrone fitting to the colour-magnitude diagrams while systematicdifferences, which are discussed on here, exist with respect to the agecalibration by Meurer, Cacciari and Freeman (1990). The present agecalibration C(15-31) vs. log(t), provided in an analytical form, isfinally used to determine the ages of the 29 clusters in our sample,including 13 objects for which no determination was available via theisochrone fitting method.
| Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.
| Blue-violet spectral evolution of young Magellanic Cloud clusters
We study the integrated spectral evolution in the blue-violet range of97 blue star clusters in the Magellanic Clouds, from those associatedwith gas emission to those as old as a few hundred Myr. Some clustersare dominated by the flux of those massive stars that pass throughevolutionary stages such as Wolf-Rayet, Luminous Blue Variable, Be, andsupergiant stars of different temperatures. The relationships amongspectral features such as absorption and emission lines, Balmerdiscontinuity and Balmer continuum are used to study the spectralevolution of the clusters. Finally, we sort into groups spectra ofsimilar evolutionary stages, creating a template spectral library withpossible applications in stellar populations syntheses of star-forminggalaxies and in the spectral simulation of bursts of star formation withdifferent mean ages and durations.
| Ultraviolet spectral evolution of star clusters in the IUE library.
The ultraviolet integrated spectra of star clusters and H II regions inthe IUE library have been classified into groups based on their spectralappearance, as well as on age and metallicity information from otherstudies. We have coadded the spectra in these groups according to theirS/N ratio, creating a library of template spectra for futureapplications in population syntheses in galaxies. We define spectralwindows for equivalent width measurements and for continuum tracings.These measurements in the spectra of the templates are studied as afunction of age and metallicity. We indicate the windows with a strongmetallicity dependence, at different age stages.
| Analysis of the UV spectra of young clusters of the Large Magellanic Cloud
UV and visual spectral energy distributions of young generations havebeen synthesized from evolutionary tracks in the HR diagram and modelatmospheres. The influence of several parameters has also been analyzed.Their UV portions have been checked with the UV spectra of 24 clustersof the LMC obtained by Cassatella et al. (1987). The models generallyagree well with the observations and make it possible to derive ages forvarious assumed abundances. Ages thus derived have been compared, in anage-age diagram, with those obtained from the integrated UBV photometry.While there are no systematic differences between these two agedeterminations, a fraction of clusters displays a large scatter, largerthan what is expected from the observational errors alone. Possiblecauses for this scatter are briefly analyzed.
| Ultraviolet colors as age indicators for LMC clusters
Empirical correlations are found between log ages and the intrinsicultraviolet colors for 27 LMC clusters. The problems and limitations ofusing these correlations as age indicators for LMC clusters and otherstellar populations are discussed. The correlations are used to estimatethe ages of two LMC clusters of unknown age (NGC 1968 and NGC 1974) andthe nuclei of two nearby blue compact dwarf galaxies (NGC 1705 and NGC5253). For the latter two objects optical- and ultraviolet-based ageestimates are in good agreement.
| The age calibration of integrated ultraviolet colors and young stellar clusters in the Large Magellanic Cloud
Integrated colors in selected far-UV bands are presented for a largesample of Large Magellanic Cloud (LMC) clusters. Theoreticalcalculations of these integrated colors are derived and discussed. Thelocation in the two-color diagram C(18-28), C(15-31) is expected to be asensitive but smooth function of cluster age for ages in the range 5 to800 million yr. Theoretical results appear in very good agreement withthe observed colors of LMC clusters. From this comparison, the gap inthe observed colors is suggested to be caused by the lack of LMCclusters in the range of ages between 200 million to one billion yr. Thetwo-color location of old globulars is discussed, also in connectionwith available data for the M31 clusters.
| LMC clusters - Age calibration and age distribution revisited
The empirical age relation for star clusters in the Large MagellanicCloud presented by Elson and Fall (1985) are reexamined using ages basedonly on main-sequence turnoffs. The present sample includes 57 clusters,24 of which have color-magnitude diagrams published since 1985. The newcalibration is very similar to that found previously, and the scatter inthe relation corresponds to uncertainties of about a factor of 2 in age.The age distribution derived from the new calibration does not differsignificantly from that derived in earlier work. It is compared with agedistributions estimated by other authors for different samples ofclusters, and the results are discussed.
| The ultraviolet spectra of M31 globular clusters
Ultraviolet spectra of 11 of the brightest globular clusters in M31 showthat some exhibit residual flux below 3000 A, greater than that expectedfrom the bright, evolved stars in the cluster. There seems to be noapparent correlation of the strength of this ultraviolet flux withparameters such as metallicity, U-B color, visual magnitude, X-rayemission, or location within the parent galaxy. However, comparison ofthe ultraviolet colors of the M31 globular clusters with those in theGalaxy and in the Large Magellanic Cloud suggests that the M31 clustersmay contain a high percentage of blue horizontal-branch stars or thatsome clusters could be as young as about 2 x 10 to the 9th yr.
| Ultraviolet observations by the IUE of 31 clusters of the Large Magellanic Cloud
Ultraviolet observations of 31 star clusters of the Large MagellanicCloud obtained by the International Ultraviolet Explorer are presented.The clusters, mostly globular, span the range of ages from about 10 tothe 7th to 10 to the 10th yr. The sample includes several young globularclusters, which have no counterpart in the Galaxy. The paper is devotedto the study of the basic properties of the clusters in the ultraviolet,such as the interstellar reddening, the spatial extension in theultraviolet, and the ultraviolet color indices. These data are essentialfor a later study of the evolutionary stage of the clusters. Oneimportant result of this study is that a discontinuity exists in the UVtwo-color diagram, a fact which deserves further investigation, since itmight support the suggestion made by other authors that an active starformation phase took place in the Large Magellanic Cloud starting about100 million yr ago.
| Age calibration and age distribution for rich star clusters in the Large Magellanic Cloud
An empirical relation is presented for estimating the ages of rich starclusters in the Large Magellanic Cloud (LMC), to within a factor ofabout 2, from their integrated UBV colors. The calibration is based onpublished ages for 58 LMC clusters derived from main-sequencephotometry, integrated spectra, or the extent of the asymptotic giantbranches. Using stellar population models, a sample of LMC clusters moremassive than about 10,000 solar masses is isolated, which is correctedfor incompleteness as a function of magnitude. An unbiased agedistribution for three clusters is then determined. The number ofclusters decreases with increasing age in a manner that is qualitativelysimilar to the age distribution for the open clusters in our Galaxy. TheLMC age distribution is, however, flatter, and the median age of theclusters is greater. If the formation rate has been approximatelyconstant over the history of the two galaxies, then the age distributionobtained here implies that clusters are disrupted more slowly in theLMC. The results contain no evidence for bursts in the formation ofclusters, although fluctuations on small time scales and slow variationsover the lifetime of the LMC cannot be ruled out.
| A Catalogue of Clusters in The LMC
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